Heterodyne Crystal Failures Mode?

[Jim Flanagan]

Please excuse me if this accidentally was double posted....

Hi guys..
Well I spent some time this weekend looking into this xtal issue.  The
HW-101 that I am restoring had three crystal that would not oscillate:

22.895 MHz ( 14 MHz Band)
37.385 MHz ( 28.5 MHz Band)
37.895 MHz ( 29 MHz Band)

What I ended up doing is to take each one out, one at a time, and make a
quick series resonant measurement using a sig gen and scope.  I could tell
that there was a 'slight' resonance on each that was somewhat 3-4 kHz higher
than it should have been.  I also know that these xtals used in a parallel
mode, but the series resonance was easier for me to check and once the
series res was found then the parallel res would/should be about 1.5 kHz or
so higher.

Thus, verifying that the crystals *appeared* dead, I used a heat gun to melt
the solder around the base and remove the cover.  Under a jewelers loop
there were no discernible fractures, etc.  I made a simple continuity test
on each leg and all looked OK.  Now the magic.  I used some 99.9% pure
alcohol with a q-tip and gently cleaned each side of the quartz material.
Using another Q-tip, dry, I went back over the quartz material to help dry
out the device.  Upon a recheck using my sig gen/scope, the xtal was again
functioning!  I carefully resoldered the case, checked again and then
reinstalled the xtal back into the unit.  Worked like a champ!  Of course,
time will tell how long the fix is good for.

I then repeated the same procedure for the other two xtals.  Each time with
success. So, the question is, WHY??  Are the xtals subjected to residue that
caused them to deteriorate?  When I cleaned them, there was no noticeable
residue, etc.  I notice that there is a small pin hole on the side of each
xtal that is soldered shut.  Do they pull a vacuum on xtals?  If not, what
is the hole for?

Well, there you have it.  I guess the next time a xtal fails, I will try
this trick to see if I can bring it back to life.

Your comments & criticism are welcome..
Jim
WB5KYE

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